1. Field of the Invention
The present invention generally relates to a data writing and reading method for a memory and, more particularly, to a data writing and reading method for writing and reading image data in a frame memory of a graphical display apparatus.
2. Description of the Related Art
A graphical display apparatus generally uses a frame memory to store image data therein. The image data includes dot data corresponding to each pixel obtained by scanning an image. The image data is written in the frame memory, and is read from the frame memory, when it is needed, by a predetermined sequence so that the dot data output from the frame memory is arranged in a predetermined sequence. Recently, in order to increase processing speed of image data, methods are suggested for increasing an access speed to the frame memory and increasing an amount of data which can be accessed by a single access operation. The amount of data accessible by a single access operation can be increased by increasing a width of a bus line.
However, in the method in which the width of the bus line is increased, the number of dots accessed at the same time is increased. Thus, there is a problem in that access is made to unnecessary dot data stored in the frame memory when an access is required for the dot data corresponding to only a small area.
Generally, the frame memory comprises a dynamic random access memory (DRAM). An operation of the DRAM requires a precharge operation. That is, in the operation of the DRAM, an access cannot be made immediately after a previous access has ended since a precharge operation must be performed before each reading operation. This increases an access interval for the frame memory comprising the DRAM. That is, a precharge period needed for the precharge operation is an obstacle to achieve a high speed access to the frame memory to provide an improved image drawing characteristic.
Japanese Laid-Open Patent Application No. 59-149391 discloses a high speed writing method for a frame buffer. In this method, a series of point data is written in the frame buffer. The point data comprises a series of dots represented by vector components obtained by a digital differential analyzer (DDA). The point data is stored in a DDA buffer having a predetermined storage capacity before it is written in the frame buffer. The frame buffer is divided into a plurality of portions each having a storage capacity equal to the storage capacity of the DDA buffer so as to write the data of the DDA buffer to the frame buffer by a single operation. However, this patent document is not directed to a concept of the data writing and reading method performed with a frame memory which is divided into a plurality of portions with each of the parts having a plurality of banks.